non-const references as function arguments

Question

I have written the following code . I worte two functions increase and constincrease which take class references as input . constincrease takes it as constant while increase is simpler and allows it to be modified .

i do not understand why line fails to compile

increase(fun()); // this line fails to compile

.. I know temporary objects are constants and hence the above line can fail as increase takes a non-const reference . However if we see there is another line of code that passes

increase(x3);

Point is why the above line passes but something else fails to compile properly.

class X{
      public:
          int i ;
      };

X fun(){
      return X();
      }

void  increase(  X & p ) 
      {
        cout<< "\n non-const-increase called ";
    p.i++;
      }

void  constincrease(  const X & p ) 
      {
        cout<< "\n const-increase called ";                   
        // this function p is const so   p.i++; is wrong ...
      }

int main(int argc, char *argv[])

{

    X x3;
    x3.i=9;
    increase(x3);  // this line passes compilation 
    constincrease(x3); // const-increase alwasy passes

    increase(fun()); // this line fails to compile 
    constincrease(fun()); // const -increase passes
    system("PAUSE");
    return EXIT_SUCCESS;
}